Quartz crystals are widely used to control the frequency of oscillation in crystal controlled oscillator. The so-called quartz crystal microbalance is well suited for film thickness monitors in vacuum deposition processes. In general, measurements are carried out by monitoring changes in the resonant frequency of the oscillating crystal. For this purpose monitors usually have at least two circuits, first an oscillator circuit to drive the crystal at resonance and, second, a measuring circuit to monitor the frequency of the crystal and thus its change in frequency changed by the deposited or etched film. See "Workshop Notes and Short Contributions," Vacuum, Volume 32, number 5, pp. 305-307, 1982, for a description of etching and deposition monitoring with an oscillating quartz crystal. See also the Technical Note number 002, published Dec. 19, 1971 by Kronos, Inc., entitled "QM-300 Series Scale Factor Considerations" describing the basic considerations of quartz crystal microbalances.
In conventional reactor systems a major source of error is temperature changes of the crystal caused by radiation occuring within the evaporator. Moreover, errors can occur through the stress effect caused by the manner in which the crystal is mounted within the reactor, particularly during high temperature ambients. Typically, a crystal is mounted under spring tension to establish electrical contact with one surface of the oscillator and to maintain it in position within the housing. At high temperatures, greater than 200.degree. C., for example, the metallic spring electrodes tend to deform and effect undesirable stress on the crystal and thereby alter its resonant frequency. Moreover, in certain reactors, such as glow discharge reactors, both a.c. and d.c. energized, the r.f. fields within the chamber of the reactor adversely affect the resonant frequency of the crystal.
There is a need in the art of crystal-controlled oscillators to minimize the stress effects induced at high temperatures on the crystal and also to reduce the radio frequency effects in reactors in which at least stray radio frequency fields are generated.